26. The Laplace transform of i(t) is given by i(s) = 2/s(1 + s). as t the value of i(t) tends to
(a) 0
(b) 1
(c) 2
(d) 0
- The Fourier series expansion of a real periodic signal with fundamental frequency fo is given by gp(t) = c0ej2nft . it is given that c3 = 3 + j5.
(a) 5 + j3
(b) -3 – j5
(c) -5 + j3
(d) 3 – j5
- Let x(t) be the input to a linear, time-invariant system. the required output is 4x(t – 2). the transfer function of the system should be
(a) 4ej4t
(b) 2e-j8t
(c) 4e-j4t
(d) 2ej8t
- A sequence x(n) with the z-transform x(z) = z4 + z2 – 2z + 2 – 3z-4 is applied as an input to a linear, time-invariant sysyem with the impulse response h(n) = 2(n – 3), where (n) = {1, n = 0 0, otherwise
the output at n = 4 is
(a) -6
(b) zero
(c) 2
(d) -4
- Convolution of x(t + 5) with impulse function (t – 7) is equal to
(a) x(t – 12)
(b) x(t + 12)
(c) x(t – 2)
(d) x(t + 2)
- Which of the following cannot be the fourier series expansion of a periodic signal?
(a) x(t) = 2 cos t + 3 cos 3t
(b) x(t) = 2 cos t + 7 cos t
(c) x(t) = cos t + 0.5
(d) x(t) = 2 cos 1.5t + sin 3.5 t
- The fourier transform f(e-t u(t) is equal to 1/1 + j2f. therefore, f{1/1 – j2t} is
(a) ef u(f)
(b) e-f u(f)
(c) ef u(-f)
(d) e-f u(-f)
- A linear phase channel with phase delay tp and group delay tg must have
(a) tp = tg = constant
(b) tp f and tg f
(c) tp = constanf and tg f
(d) tp f and tg = constant
- Conider a sampled signal :
y(t) = 5 x 10-6 x(t) (t – nts),
where, x(t) = 10 cos (8 x 103) t
and ts = 100 us. when y(t) is passed through an ideal low-pass filter with a cut-off frequency of 5 khz, the output of the filter is
(a) 5 x 10-6 cos (8 x 103) t
(b) 5 x 10-5 cos (8 x 103) t
(c) 5 x 10-1 cos (8 x 103) t
(d) 10 cos (8 x 103) t
- The transfer function of a system is given by h(s) = 1/s2 (s – 2) . the impulse response of the system is (* denotes convolution, and u(t) is unit step function)
(a) (t2 * e-2t) u(t)
(b) (t * e2t) u(t)
(c) te-2t) u(t)
(d) (te-2t) u(t)
- The region of convergence of the z-transform of a unit step function is
(a) |z|>1
(b) |z|<1
(c) (real part of z)> 0
(d) (real part of z) < 0
- Let (t) denotes the delta function. the value of the integral (t) cos (3t/2) dt is
(a) 1
(b) -1
(c) 0
(d) 2
- A band limited signal is sampled at the nyquist rate. the signal can be recovered by passing the samples through
(a) an R-C filter
(b) an envelope detector
(c) a PLL
(d) an ideal low-pass filter with appropriate bandwidth
- A linear time-invariant system has an impulse response e2t, t > 0. if the initial conditions are zero and the input is e3t, the output for t > 0 is
(a) e3t – e2t
(b) e5t
(c) e3t + e3t
(d) none of these
- Given that l[f(t)] = s + 2/s2 + 1, l[g(t)] = s2 + 1/(s + 3) (s + 2). h(t) = f(r) g(t) d. l[h(t)] is
(a) s2 + 1/s + 3
(b) 1/s + 3
(c) s2 + 1/(s + 3) (s + 2) + s + 2/s2 + 1
(d) none of these
- The fourier transform of the signal x(t) = e-3t2 is of the following form, where A and B are constants
(a) Ae-5(t)
(b) Ae-bf2
(c) A + B |F|2
(d) Ae-bf
- A system with an input x(t) and output y(t) is described by the relation y(t) = tx(t). this system is
(a) linear and time-invariant
(b) linear and time-variant
(c) non-linear and time-invariant
(d) non-linear and time-variant
- The n-point discrete fourier transform of a signal x[n] is xdft [k]. if DFT of another discrete sequence y[n] is given as ydft [k] = x*dft [k], then which of the following is true?
(a) y[n] = x* [n]
(b) y[n] = nx* [n]
(c) y[n] = 1/n x* [n]
(d) y[n] = – x* [n]
- Discrete fourier transform of a real sequence {x[n]} is given as
xdft[k] = {0, a, 2 + j, – 1,b,j}
then, a and b are
(a) 2 – j, j
(b) j, -j
(c) -j, 2 – j
(d) j, 2 + j
- The 4-point discrete fourier transform of a discrete time sequence (1,2,1,2) is
(a) {0, -2, 0, 6}
(b) {6, -4j, 6 – 4j}
(c) {-4j , 6 – 4j, 6}
(d) {6, 0, -2, 0}
- Consider a band-pass signal xc (t) is shown below. the minimum sampling rate required for this signal to prevent loss of information is
(a) 4 kHz
(b) 12 kHz
(c) 20 kHz
(d) 2 kHz
- Two signals x1(t) are band-limited to 2 kHz and 3kHz respectively, then nyquist rate for the signal x1(t)* x2(t) is
(a) 5 kHz
(b) 6 kHz
(c) 4 kHz
(d) 10 kHz
- Which one is most appropriate dynamic system out of the following?
(a) y(n) = y(n – 1) + y(n + 1)
(b) y(n) = y(n – 1)
(c) y(n) = x(n)
(d) y(n) + y(n – 1) + y(n + 3) = 0
- Which one is causal system?
(a) y(n) = 3x(n) – 2x(n – 1)
(b) y(n) = 3x(n) + 2x(n + 1)
(c) y(n) = 3x(n + 1) + 2x(n – 1)
(d) y(n) = 3x(n + 1) + 2x(n – 1) + x (n)
- For the signal given below
y(t) = 2 sin (2/3 t) + 4sin (1/4 t 4) + 6 sin (1/3 t 5) + 8 sin (1/2 t 7)
the common period of y(t) is given by
(a) 12
(b) 24
(c) 8
(d) 16
- (c)
from final value theorem,
lim I(t) = lim sI (s)
= lim s x 2/s (1 + s)
= 2
- (d)
the coefficients cn and c-n in the fourier series are complex conjugate.
C-N = CN*
|C-N |= |CN|
<C-N = – <CN
C3 = 3 + J5
C-3 = C3* = 3 – J5
- (c)
transfer function h(s) = FT of output/FT if input
= 4e-j20 x(s)/x(s) = 4e-j2o
h(s) = 4e-j4f
- (b)
y(z) = h(z) x(z)
given, x(z) = z4 + z2 – 2z + 2 – 3z-4
h(n) = [n – 3]
h(z) = z-3
y(z) = z-3 [z4 + z2 – 2z + 2 – 3z-4]
= z + z-1 + 2z-2 + 2z-3 – 3z-7
taking inverse z-transform,
by inspection,
y[n] = [n + 1] + [n – 1] – 2[n – 2] + 2 [n – 3] – 3 [n – 7]
putting n = 4
y[4] = 0
- (c)
from shifting property of impulse function,
(t)* (t – to) = (t – t0)
hence, x(t + 5)* (t – 7) = x(t + 5 – 7)
= x(t – 2)
- (b)
signal x(t) = 2 cos t + 7 cos t
period of 2 cos t = t1 = 2/0
= 2 = 2
period of 7 cos t = t2 = 2/0
= 2/1 = 2
for signal x(t) to be periodic t1/t2 not be rational.
t1/t2 = 2/2 = 1 = irrational
periodic waveform is sufficient condition for existence of fourier series. hence, x(t) = 2 cos t + 7 cos t is not a periodic signal hence, fourier series expansion is not possible.
- (c)
from duality property of fourier transform.
x(t) – x(f)
x(t) – x(-f)
f{1/1 + j2t} = ef u(-f)
- (a)
let VI = x(t) cos t is applied in linear phase channel.
where, Tg = group delay
Tp = phase delay
in liear phase channel, transmission is distrtionless.
hence, TP = TG = constant
- (c)
given that sampling interval ts = 100 us
sampling frequency fs = 1/ts = 10 kHz
bandwidth of LPF = fc = 5 kHz
fs = 2 B
hence, sampled output not overlap.
filter output = y(t)/ts
= 5 x 10-6 x x(t)/100 x 10-6
given, x(t) = 10 cos (8 x 103) t
output = 5 x 10-6 x 10 cos (8 x 103) t/100 x 10-6
= 5 x 10-1 cos (8 x 103) t
- (b)
transfer function h(s) = 1/s2(s – 2)
lmpulse response h(t) = l-1 [h(s)]
= l-1 [1/s2(s – 2)]
we know that
l-1 [f1 (t). f2(t)] = l-1 [f1(t)* l-1[f2(t)]
hence, h(t) = l-1 [1/s2]* l-1|1/s – 2]
= t u(t)* e2t u(t)
= (t* e2t) u(t)
- (a)
x[n] = u[n]
x[z] = x[k] z-k
u[k] z-k = z-k
x[z] = 1/1 – z-1
for x[z] to converge,
|z-1|< 1
|z|> 1
- (a)
from sampling property of unit impulse function,
(t) (t) dt = (0)
(t) cos (3t/2) dt = cos (0) = 1
- (d)
As band limited signal is sampled at nyquist rate. hence, replicated spectra do not overlap and the original sepctrum can be generated by passing 0s (t) through ideal low-pass filter that has cut-off frequency.
(bandwidth fc = fs/2 ; where fs is sampling frequency.
- (a)
y(s) = x(s) h(s)
given, x(t) = e3t, h(t) = e2t
x(s) = 1/s – 3, h(s) = 1/s – 2
y(s) = x(s) h(s) = 1/(s – 3) x 1/(s – 2)
= 1/s – 3 – 1/s – 2
hence, y(t) = e3t –– e2t
- (b)
given, l[f(t)] = s + 2/s2 + 1 = f(s)
l[g(t)] = s2 + 1/(s + 3)(s + 2) = g(s)
h(t) = f g(t-) d
h(t) = f(t)* g(t)
taking laplace transform,
l[h(t) = l[f(t)* g(t)]
= l[f(t)]. l[g(t)]
= f(s). g(s)
= s + 2/s2 + 1 x s2 + 1/(s + 3) (s + 2) = 1/s + 3
- (b)
the signal x(t) = e-3t2 is an example of gaussian distribution. the fourier transform of gaussian distribution is also caussian distribution.
x(f) = A e-bf2
- (b)
let x(t) = a x1 (t) + b x2 (t)
then, a y1 (t) + b y2 (t) = at x1 (t) + bt x2(t)
= t x(t)
then system is linear.
if input is delayed by t0, i.e., x(t – t0)
then output is
y1 (t) = t x (t – to)
but output is delayed by to.
y2(t) = (t – to x (t – to)
y1 (t) = y2(t)
hence, system is not time-invariant, i.e., time varying.
- (b)
x[n] = xdft [k]
a typial DFT term XDFT[K] has the form Aejo
let = 2kn /n, the IDFT gives x[n] with terms of the form
1/n Aejo . ej = 1/n Aej(0+ 0)
if we conjugate XDFT [k], typical term becomes Ae-jo. its IDFT gives terms of the form Ae-joe-jo = Ae-j(0 + 0)
this DFT result corresponds to the signal Nx *[n].
- (c)
the DFT of a real signal shows conjugate symmetry as
XDFT[K] = XDFT [N – K]
here, N = 6
a = XDFT [1] = XDFT [6 – 1] = XDFT [5]
a = – j
B = XDFT [4] = XDFT[6 – 4] = XDFT[2]
B = 2 – J
- (d)
let x[n] = (1,2,1,2} is discrete fourier transform of x[n] is given as
XDFT [K] = X[n] e-j2nk/n
k = 0, 1,……….N – 1 and
n = 4
so, XDFT [K] = x[n] e-j2nk/2
x[n] e-jkn/2
k = 0, XDFT [0] = x[n] eo
x[0] + x[1] + x[2] + x[3]
= 1 + 2 + 1 + 2 = 6
k = 1, XDFT [1] = x[n] e-jn/2
= x1 [0] e0 + x[1] e-j/2 + x[2] e-j + x[3] ej3/2
= 1 – 2j – 1 + 2j = 0
k = 2, XDFT [2] = x[n] e-jn
= x[0] e0 + x[1] e-j + x[2] e-j2 + x[3] e-j3
= 1 – 2 + 1 – 2 = 2
k = 3, XDFT [3] = x[n] e-jn3/2
= x[10] e0 + x [1] e-j3/2 + x[2] e-j3 + x[3] e-j9/2
= 1 – 2j – 1 + 2j = 0
so, XDFT [K] = {6,0, -2, 0}
- (a)
for a band-pass signal, minimum sampling frequency is given as
fs = 2fH/N
where, N = int (fH/B)
Here, fl = 4 kHz, fh = 6 kHz
bandwidth B = FH – FL = 6 – 4 = 2 kHz
N = int (6/2) = 3
fs = 2 x 6/3 = 4 kHz
- (c)
x1 (t) * x2(t) = x1 (f) x2 (f) (product of spectrum)
so, x1(t)* x2(t) extends only to 2 kHz
nyquist rate = 2 x 2 kHz = 4 kHz
- (a)
Because present output y(n) depends upon past output y(n – 1) and future output y(n + 1).
- (a)
for causal system output must depend upon present and past, not on future.
- (b)
The periods of individual components are
01 = 2/3 2/T1 = 2/3 t1 = 3
02 = 1/4 2/t2 = 1/4 t2 = 8
03 = 1/3 2/t3 = 1/3 t2 = 6
04 = 1/2 2/t4 = 1/2 t4 = 4
Common period is LCM of the individual periods
T = LCM (3, 8, 6, 4)
= 24